Digital data communication over a wireless channel is used in a variety of applications. One way to encode digital information in an RF signal is by manipulating the phase of the signal in dependence on the digital information which is to be transmitted. At the receiving end the phase changes are detected and the digital information is reconstructed. Transmitting data by encoding the information in phase changes is referred to as phase-shift keying (PSK). For example, it conveys data by modulating the phase of a reference signal (the carrier wave).
Phase-shift keying (PSK) is used in a variety of applications such as: communication to and from RFID tags, wireless LAN, including Wi-Fi, and Bluetooth. PSK may use a finite number of phases to represent digital data, also called symbols. Each phase may be assigned a unique pattern of binary digits.
At the transmitter side, a modulator maps digital information to a sequence of symbols which in turn is encoded in the phase changes of a signal which is to be transmitted using an antenna. At the receiver end, a demodulator determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original digital information. Typically, the demodulator is designed specifically for the symbol-set used by the modulator.
The phase may be defined with respect to a reference signal, or with respect to a previous phase. In the first case, the demodulator of the receiver compares the phase of the received signal to the reference signal. In the latter case, the demodulator determines the changes in the phase of the received signal rather than the phase relative to a reference wave; this approach is termed differential phase-shift keying (DPSK) since it depends on the difference between successive phases.
When communicating over a wireless channel, one of the sources of error on the received data is inter-symbol interference (ISI), meaning that received symbols are affected by surrounding (typically past) transmitted symbols, for example due to a bandwidth limited physical channel. One of the methods to reduce ISI is to pre-equalize the signal applied to the channel on the transmitter side such that after passing through the channel, the ISI is zero. The intended signal, i.e., the phase modulated signal encoding the digital information, is modified before it is broadcasted over the antenna, anticipating the distortions that are going to occur due to inter-symbol interference. In the ideal case, the signal is received exactly as intended since the distortion that occurred exactly cancelled the pre-equalization.
One approach to pre-equalization is to apply a linear filter with a transfer that is the inverse of the channel transfer, so that the combination of filter plus channel is ISI free. This technique is referred to as zero-forcing equalization. A known signal distortion pre-compensation process is given in US patent application 2010/0134255 A1, “RIFD device, RFID system and signal distortion pre-compensation process in RFID system”, incorporated herein by reference. The known signal distortion pre-compensation process is applied to an RFID system. An RFID device comprises an interface for transmitting wireless carrier and data signals to a remote RFID transponder. Carrier and data signal pre-compensation means are arranged between the antenna and the data signal processing means of the RFID device. The carrier and data signal pre-compensation means are adapted to pre-compensate signal distortions of the carrier and data signals caused by quality factors of the device air interface and the transponder air interface of the RFID device and the RFID transponder, respectively.